This invention relates to dies suitable for the plastic working for cylindrical parts including metallic tubes and the like.
As an extrusion method for metallic tubes, there has been known such a method that a billet is inserted into a container installed with dies, a small diametral portion (a mandrel) of a punch solidly secured to a press slide is inserted from one side of the billet, thereafter the slide is lowered, the billet is pressed into the dies by use of the large diametral portion of the punch, the billet is subjected to ironing along the outer diameter thereof to be formed into a tubular product, and the product is removed as the slide is elevated.
Heretofore, dies which have been adopted in this field of work include a curved surface die having a curved surface of a relatively large curvature in longitudinal cross-section and a conical shape die having a cross-section of a combination of substantially straight lines from the shoulder portion to the relief portion of the die with the vertex of said each line being at the ironing portion.
The former tends to cause seizing because of increased extrusion pressure due to a large contact area between the billet and the dies at the ironing portion. Consequently, the extrusion of a material having high strength may result in breakage and wear of the dies and is impracticable.
Additionally, with the conical shape dies, a high extrusion pressure is required for a material of high strength, which may result in breakage and wear of the dies. These shortcomings can be obviated by making the relief portion small. However, as the contact area is increased due to the wear of the ironing portion, increased extrusion pressure comes to be required, thus resulting in seizing and breakage of the dies. Further, a large lateral pressure acts on the dies, thus resulting in decreased finishing accuracy of the product.